Process and apparatus for eliminating short circuits in electrical discharge machining

Abstract

Process and apparatus for eliminating short circuits occurring in the machining zone between an electrode workpiece and an electrode tool in the course of an electrical discharge machining operation. Short circuits, caused by bridging of the electrodes by metallic particles, are burnt by an auxiliary high intensity electrical discharge across the gap between the electrodes applied after a predetermined time interval, or time delay, from the beginning of a normal current pulse.

I claim: 
     
       1. In a process for machining an electrode workpiece by an electrode tool wherein consecutive voltage pulses are applied across said electrodes for causing current pulses of a predetermined intensity to flow thereacross, comprising monitoring the machining conditions in the machining gap between the electrodes and increasing the current pulse intensity when such monitoring indicates the presence of a short circuit bridging the gap, the improvement comprising increasing the intensity level of said current pulse after a first predetermined time interval delay from the beginning of said current pulse, interrupting the increased intensity level of said current pulse after a second predetermined time interval delay following said first time interval delay, monitoring the machining conditions during said second time interval delay and reestablishing current pulses of normal intensity as soon as such monitoring indicates the disappearance of said short circuit. 
     
     
       2. The improvement of claim 1 wherein the intensity of said current pulse is increased as a function of time. 
     
     
       3. The improvement of claim 2 wherein said intensity is increased in predetermined steps at predetermined intervals in the course of said current pulse. 
     
     
       4. In an apparatus for machining an electrode workpiece by means of an electrode tool wherein consecutive voltage pulses are applied across a machining zone between said electrode workpiece and said electrode tool for causing current pulses of predetermined intensity to flow thereacross, said apparatus comprising a pulse generator for applying between the electrodes said consecutive voltage pulses separated by a predetermined time interval, means for developing a first electrical signal corresponding to the passage of machining current across the machining zone, means for measuring at least one electrical value representing the machining conditions in the machining zone, means for developing a second electrical signal when said electrical value reaches a predetermined level indicating the presence of a short circuit in said machining zone, at least one switching means for increasing the current intensity across said machining zone, and means for controlling said switching means when said first and second electrical signals appear, the improvement comprising first delay means associated with said first electrical signal for activating said switching means for increasing the current intensity across said machining zone after a first predetermined time interval from the beginning of a current pulse, second delay means deactivating said switching means for decreasing the current intensity across said machining zone after a second predetermined time interval following said first time interval, and means controlled by said second electrical signal for deactivating said switching means upon disappearance of said second electrical signal. 
     
     
       5. The improvement of claim 4 further comprising at least one auxiliary current source, and wherein said switching means is connected in series between said auxiliary current source and said electrodes. 
     
     
       6. The improvement of claim 5 comprising at least two auxiliary current sources and means controlled by said delay means for selectively connecting each of said auxiliary sources to the machining gap at predetermined moments in the course of a current pulse. 
     
     
       7. The improvement of claim 4 wherein said auxiliary sources comprise means for charging a capacitor and means for discharging said capacitor in parallel to said pulse generator, said means for discharging said capacitor being controlled by said switching means.